more refactoring.

I did more refactoring. It is following.

* add `scriptencoding utf-8`
* `if !exists("...") let... endif` -> `let g:... = get(g:,...)`
* Do not abbreviate. (e.g. `exec` -> `execute`)
* add `augroup indentLine`
* `==` -> `is`, `!=` -> `isnot`, ...
* Use setlocal. (e.g. `set conceallevel=1` -> `setlocal conceallevel=1`)